In a spread spectrum communication system, the spread spectrum signal may be formed by mixing a narrowband information signal with a pseudonoise (PN) code sequence, and then phase modulating this mixed signal. One such phase modulating technique is minimum shift keying (MSK). MSK signal modulation is popular since signals produced thereby have nearly continuous amplitude envelopes and low spectral sidelobes as compared to similar bi-phase (or quadrature) shift keyed (BPSK) signals.
In MSK systems, effective recovery of the narrowband information signal at the receiver requires synchronization between the received spread spectrum signal's PN code sequence and a local PN code sequence incorporated in a reference signal with which the receive signal is correlated. Surface acoustic wave devices have been used in such systems to convolve the received spread spectrum signal and the reference signal. Such devices typically include first and second transducers formed on the top surface of a piezoelectric substrate for producing surface acoustic waves in response to the received spread spectrum signal and reference signal applied thereto. Typically, the convolver includes a conductor material placed intermediate of the first and second transducers. The transduced input and reference signals are combined under the conductor, where crystal non-linearity produces product signals that are then spatially integrated by the conductor to produce a convolution of the signals.
To effectively convolve the input and reference signals, these signals must utilize the same modulation scheme. In prior art systems, the reference signal was formed by mixing a reference PN code sequence with a local oscillator signal to produce a bi-phase shift keyed signal. This BPSK signal was then applied to an external surface acoustic wave device which converted the signal to an MSK form and then back to the electrical domain.
External transformations between the acoustical and electrical domains are disadvantageous since they degrade the signal and increase the power consumption and cost of the receiver. Therefore, there is a need to provide a surface acoustic wave convolver for use in a receiver of an MSK communication system which obviates these additional external transformations.